Save Money and Improve Productivity and Machine Reliability with Pneumatic Actuators

“A common goal throughout the business world is to increase productivity while reducing costs. This certainly applies to machine design, which means achieving the fastest possible cycle times. For pneumatic systems, this often requires running actuators as fast as possible without introducing excessive shock loads to the system.” This next series of blogs are from Michael Guelker, product manager – pneumatic actuators, with Festo USA.  I think that you will find the topic quite interesting.

To help reduce the impact energy of an actuator, there are two common types of integrated cushioning – bumpers and air cushioning. Flexible bumpers are generally made of an elastomer material and incorporated as part of the piston or end cap. They are effective at reducing impact noise, however they can only absorb small amounts of energy so they are limited to slow speed, small load, or short stroke applications. When operating with higher loads and speeds, you must use additional cushioning in order to avoid premature damage to the cylinder and machine.

Designers generally use air cushioning for end-of stroke damping in these situations. The air cushioning needle valve needs to be adjusted according to the cylinder’s operating pressure, speed, and load. This requires the time of 1 or 2 experienced maintenance people, which is costly. If there are any changes to the air pressure, speed, or load then the air cushioning needs to be re-adjusted, otherwise you may experience slower cycle times and increased shock and vibration which will cause premature failure of the cylinder and potentially other components on the machine. This reduces productivity and costs money so it would be advantageous if this can be avoided. A new air cushioning solution automatically adjusts the amount of air cushioning depending on the impact energy (speed, load), thus increasing overall productivity and reducing maintenance costs.

Before we review the details of the design and operation of the new self-adjusting air cushioning, we will look at the principles behind air cushioning and the operation of typical adjustable air cushioning.

Air cushions are variable-orifice air dashpots. A spud on either side of the cylinder piston closes off the passage to the main piston chamber. This traps air in the cylinder end cap and bleeds it off through a small passage controlled by a needle valve. The cylinder traps a fixed volume of air each cycle but because air is highly compressible, the load, velocity, and air pressure in the cylinder all affect air-cushion performance. For any given set of conditions there is generally only a small window of needle-valve adjustments which provide optimal cushioning. Closing the needle valve too far results in high initial reaction force, which slows the load too quickly. This may cause end of stroke bounce, which is when the piston changes direction multiple times as deceleration progresses, causing vibration and a longer cycle time. If the needle-valve is open too far, the trapped air does not create sufficient back pressure. The load reaches the end of stroke too quickly and causes an impact between piston and end cap. Furthermore, once the needle-valve is properly set, any change in weight, system pressure, or velocity affects cushion reaction, and means the valve must be readjusted.

Check back next week for part two in this blog series on innovative air cushion solutions for pneumatic actuators.

 

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